The battlefield is Rockfall Path, and the unit ability has play a token unit here. What happens to the token unit?

Because Rockfall Path has the ability "Units can't be played here," any attempt to play a unit (including a token unit created by an ability) to that battlefield results in an illegal action.

According to the RiftJudge FAQ (ID 6500), if a spell or ability instructs you to play a unit to Rockfall Path, you can shortcut the instruction to simply not execute. The spell or ability resolves, but the instruction to play the unit happens to no effect. The token unit is never created.

These effects can still be triggered for purposes like enabling other cards (e.g., Ravenbloom Student or Darius Trifarian), as the attempt to play the card still occurs, but the outcome is that the unit is not played.